A wireless modem is a data communication device that comprises at least two data communication ports, at least one of which is a wireless data port enabling the wireless modem to communicate with one or more other wireless modems via a radio frequency (“RF”) link. The other communication ports, each of the other communication ports may be connected to a single computer or a data network, may be wire line, such as Ethernet, USB, or RS232, or wireless. To simplify the following description, we will consider a wireless modem to have two communication ports—one an RF link for communicating with other wireless modems and the other a wire line link for communication with a single computer or a data network. Ethernet will be used as exemplary protocol for the wire line link in the following discussion. Wireless modems are typically used to link together wire line data networks that are geographically separated. The wireless modems in that case act as bridges between the wire line data networks. An example of such use is shown in FIG. 1. A first local area network 10 is connected by wire line 12 to a first wireless modem 14. Similarly a second local area network 16 is connected by wire line 18 to a second wireless modem 20. The two wireless modems 14, 20 communicate via an RF link 22, shown connecting wireless modem 14 with wireless modem 20 through the free space separating the modems 14, 20.
In order for wireless modems to be used in a variety of data networks supporting different communication protocols, and to facilitate efficient operation on their RF links, wireless modems typically use protocol layering. Regardless of the protocol used on the wired link, when a data packet is received by a wireless modem on its wired link and is to be transmitted on its RF link, the wireless modem encapsulates the wire line data packet within an RF packet containing information specific to the RF link. In essence the wire line data packet becomes the data payload for the RF data packet. The wireless modem then sends the complete RF packet on its RF link. The wireless modem that receives the packet removes the RF link specific information and then forwards the wire line portion of the packet on its wire line communication port.
A typical RF link packet 24 that would be transmitted from wireless modem 14 to wireless modem 20 in FIG. 1 using protocol layering is shown in FIG. 2. The data contained in RF link packet 24 includes:                Synch Sequence 26—data that precedes the rest of RF link packet 20 and allows any wireless modem receiving RF link packet 24 to synchronize on the incoming data and determine where the start of the remaining data is in the packet 40.        SRC 28—an address or unique identifier of the wireless modem (“the source modem”) that transmitted RF link packet 24, in this case wireless modem 14. All wireless modems have a unique address or identifier permanently written into them during manufacture.        DEST 30—an address or unique identifier of the wireless modem (“the destination modem”) that is expected to receive RF link packet 24. Since RF is a broadcast medium all wireless modems in the receiving area will receive the RF link packet 24. Only the wireless modem that has an SRC 28 that matches the DEST 30, in this case wireless modem 20, is expected to process RF link packet 24. All other wireless modems receiving RF link packet 24 are expected to discard it.        CONTROL 32—special control data for the destination wireless modem, in this case wireless modem 20. The control data may include an indication that the data payload (see below) is actually intended for the destination wireless modem, here wireless modem 20, and is not to be forwarded on to its associated local area network, here local area network 16. Other control data may indicate that additional data packets will be following or that no acknowledgement is expected in response to RF link packet 24.        ID 34—a unique identifier associated with RF link packet 24. Each time the source wireless modem transmits a packet on its RF link it increments the ID of the previous packet so that each transmitted RF link packet 24 is uniquely identified.        ACK ID 36—the ID of the last uncorrupted packet that the source wireless modem received from the destination modem.        PAYLOAD 38—the data portion of the RF link packet 24.        FCS 40—the frame check sequence. This is a value that is calculated from all of the data in the RF link packet 24, excluding the synch sequence 26 and the FCS 40. The destination modem, here wireless modem 20 recalculates the FCS 40 based on the data it receives. If the calculated value matches the value of the received FCS 40, then the payload 40 is forwarded on the wired link, in this case wire line 18, and the ID of the RF link packet 24 is stored to be used as the ACK ID 36 when next communicating with the source modem. If the calculated value of the FCS 40 does not match the value of the received FCS 40, then the RF link packet 24 is discarded without attempting to notify the source modem.        
Wireless modems are also used in point to multi-point wireless data communication networks, such as that shown in FIG. 3 and indicated generally by reference numeral 42. In point to multi-point wireless data communication network 42, a special form of wireless modem, base station 44, is connected by wire line 45 to a data network 43. Base station 44 receives data from data network 43 and transmits it to a group of wireless modems, a representative four of which are indicated by reference numerals 46, 48, 50, and 52 in FIG. 3, via RF links 54, 56, 58, 60, respectively. The wireless modems 46, 48, 50, 52 in turn send data via RF links 54, 56, 58, 60, respectively, directly to with the base station 44 (i.e., wireless modems 46, 48, 50, 52 do not communicate directly with each other). One common application of a point to multi-point wireless data communication network 42 is to connect a group of end users, each with one or more wireless modems 46, 48, 50, 52, to the Internet (an example of a data network 43) via the base station 44, which is maintained by an Internet Service Provider (ISP). The point to multi-point wireless data communication network 42 allows the ISP to provide Internet connectivity to its customers in a geographic area in a timely and cost efficient manner.
The base station 44 and its associated group of wireless modems 46, 48, 50, 52 all transmit data on the same RF frequency (RF channel). Only one of the base station 44 and its associated group of wireless modems 46, 48, 50, 52 can be transmitting data at a particular time in a geographic area. If more than one of wireless modems 46, 48, 50, 52 attempts to transmit data simultaneously, then all transmissions will be corrupted, i.e., they will interfere with each other, and the base station 44 will not be able to decipher any of the transmissions. In order to prevent communications from multiple modems from interfering with each other some method of coordinating access to the RF channel is required. For example, RF links 54, 56, 58, and 60 shown in FIG. 3 all share the same RF channel, but only one of them can be active at a time if interference is to be avoided.
Access to an RF channel can be coordinated if the base station or a wireless modem wishing to transmit data on the RF channel first listens to determine if any of the wireless modems, in the case of the base station, or the base station or any of the other wireless modems, in the case of a wireless modem, are currently transmitting. While wireless modems do not communicate directly with each other they are able to determine if another wireless modem, or the base station, is currently transmitting. When neither the base station nor any of the other wireless modems are transmitting, then the RF channel is available and data can be transmitted. This technique is commonly referred to as “collision-sense multiple access with collision avoidance” (“CSMA/CA”) and is illustrated in FIG. 4, which shows a portion of network 42. In FIG. 4, wireless modem 52 has data that it wishes to send to base station 44. However, at the same time, wireless modem 46 is already sending data over RF link 54, which uses the same RF channel that wireless modem 52 would have to use to send data to base station 44. Wireless modem 52 monitors the RF channel, so if it is able to receive the RF signal being broadcast by wireless modem 46, it waits until wireless modem 46 stops sending data to base station 44 before it attempt to send its data. The RF signal received by wireless modem 52 is indicated by reference numeral 62. While the RF signal broadcast by wireless modem 46 may radiate in directions, only RF link 54 and RF signal 62 are shown in FIG. 4.
One of the difficulties with the CSMA/CA technique is that it is possible for two or more wireless modems wishing to send data at the same time to simultaneously sense that the RF channel is free and to begin transmitting at the same time. This is referred to as a collision and results in both transmissions becoming corrupted. Collisions can also occur due to “hidden nodes”. This occurs when a wireless modem is unable to sense transmissions from one or more of the other wireless modems due to obstructions or other interference. If one of these wireless modems is currently transmitting data to the base station the hidden node is unable to detect this and may begin its own transmission to the base station, which will result in a collision at the base station. For example, in FIG. 5 an obstruction 64 exists between wireless modem 46 and wireless modem 52, preventing wireless modem 52 from receiving RF signal 62. Wireless modem 52 therefore attempts to send data over RF link 60 at the same time wireless modem 46 is sending data over RF link 54.
As the number of wireless modems and the amount of data to be transmitted increases, the likelihood of a collision increases. Collisions can cause less efficient use of the RF channel.
An improvement to the CSMA/CA technique, that is useful during heavy traffic periods, is to use reservation slots. The group of wireless modems associated with a base station is divided into subgroups. The base station broadcasts messages to the overall group indicating which subgroup may attempt communication at any given time. Since the number of wireless modems that may access the media at any given time is reduced the chances for collisions are reduced. Careful selection of the members of a subgroup can also reduce the likelihood of hidden nodes; however, as the number of wireless modems increases, or the amount of data to be transmitted increases, the likelihood of collisions still increases. The use of reservation slots is not illustrated in the drawings.
Collisions due to high traffic load or hidden nodes can be completely avoided by using polling. In polling, the base station queries (polls) each wireless modem in round robin fashion to determine if it has data to transmit. If the wireless modem has data to transmit it responds by sending the data to the base station. If the wireless modem has no data to send it returns a special packet (a “null response”) indicating that it does not wish to send data. The base station receives data or a null response from the wireless modem and then polls the next wireless modem. The sequence continues until the entire list of wireless modems has been polled and then repeats. Since only one wireless modem can be transmitting at any time there is no possibility for collision. Polling is illustrated in FIG. 6, in which base station 44 is sending data to or receiving data only from wireless modem 46 over RF link 54. Each of wireless modems 48, 50, and 52 remains inactive until it receives from base station 44 an RF packet with DEST 30 equal to its identifier.
Channel efficiency may be defined as the amount of time spent actually transmitting data compared to total available time, including time spent in overhead operations, such as polling modems that return no data or do not respond, transmitting RF link packet overhead, etc. During periods of heavy data traffic, polling may provide channel efficiency approaching 100% as almost every poll may return data if almost all modems have data to send. However, during periods of light traffic only a few wireless modems have data to transmit. In that case, each modem that has data to send is only allowed to transmit for a brief period of time and then must wait for an entire polling cycle before it can transmit more data. In the extreme cases where only one wireless modem wishes to transmit data the resulting channel efficiency will be very low if polling is used.
On the other hand, for CSMA/CA operation in an environment in which only a base station and a single wireless modem are operating, the channel efficiency may approach 100%. But as the number of active wireless modems increases the channel efficiency may become quite low. In fact, if too many wireless modems try to use the same RF channel, a condition known as channel collapse can occur, effectively rendering the RF channel unusable.
For polling operation the channel efficiency is greatest during periods of heavy traffic and may approach 100%. As the number of active wireless modems decreases the channel efficiency may decrease significantly. As an example, if there were 100 wireless modems being polled by a single base station and only one of the wireless modems were actively transmitting data the efficiency of the data channel would be roughly 1%. On the other hand if only one wireless modem was associated with the base station then the efficiency of the communication channel would be consistently close to 100%.
For example, in a typical outdoor wireless data network the distance between the base station and wireless modems might be 10 miles. Based on current technology, using direct sequence spread spectrum with a raw data throughput of 11 megabits per second, a polling operation between a base station and a wireless modem in which no data is exchanged will require approximately 0.5 milliseconds. For the same environment, if the base station and the wireless modem each transmitting the maximum allowable amount of data (e.g., 1518 octets for an Ethernet network) during the exchange, a total approximately 2.7 milliseconds will be required (1.1 millisecond for each transmission of 1518 octets). Ideally this would mean that the maximum efficiency of the RF channel is approximately 82% (2.2/2.7). If only the wireless modems were transmitting data, then approximately 1.6 milliseconds would be required, resulting in a channel efficiency of approximately 70%. If the base station were associated with 100 wireless modems, the worst-case channel efficiency (when only one wireless modem of the 100 wished to send data) would be approximately 2%.
From the discussion above it is evident that channel efficiency could be improved over both known polling methods and CSMA/CA by using polling if unnecessary polling of inactive or out-of-service modems could be reduced, particularly in situations in which traffic is generally moderate.